The present disclosure relates generally to dual-mode optical sensors, and more particularly, to dual-mode optical seekers, such as used for guiding a steerable rocket or other projectile.
The dual-mode seekers are typically located at the nose or tip of the projectile, such as in a laser-guided projectile. Conventional laser-guided projectiles typically have a silicon quadrant detector forming an illuminated-spot locating detector (ISLD). This quadrant detector has four light-detecting elements, generally of equal size, that have silicon light-absorbing regions. Because silicon is an indirect-bandgap semiconductor, the absorbing regions are very thick (generally more than 50 times larger than the wavelength of the light to be absorbed) in order to provide sufficient sensitivity. Silicon has reasonable absorption for wavelengths between 0.4 and 1.1 μm, but has negligible absorption for light of the eye-safe 1.4-1.65 μm wavelengths. Also, silicon has a fairly high absorption for the long-wave infrared (LWIR) wavelengths. Thus, a silicon quadrant detector is also not suitable for use as the ISLD for detecting laser light of eye-safe wavelengths or that is part of a dual-mode sensor having an in-line optical configuration of the ISLD and a LWIR imaging focal-plane array (FPA) detector. Thus, these known multi-mode optical seekers only detect light having a wavelength shorter than 1.1 μm. Moreover, using an optical diffuser in these seekers would degrade the image formed and result in less than satisfactory performance.
Other dual-mode seekers are known and configured as a combination of a semi-active laser (SAL) seeker and an imaging infrared (IIR) FPA detector. These seekers have an optical system with a wavelength-selective minor that separates the light for the SAL detector from the light for the IIR detector such that the light for the IIR detector does not pass through the SAL detector. Also, only the separated light that is directed to the SAL seeker passes through the optical diffuser or spreader. The spreader is positioned between the optical imaging elements and the SAL detector and is configured to spatially homogenize the laser energy and/or to increase the size of the spot of focused laser energy at the SAL detector. The spreader is, thus, placed between the secondary mirror/lens and SAL detector away from the common aperture and outside the common optical path and IR optical path to the IR detector.
Other prior art multi-mode optical seekers use the same FPA for detecting the SAL radiation of one wavelength and for imaging at another wavelength. One portion of the FPA of detector elements is used to detect the SAL radiation and another portion is used to detect the radiation to be imaged. These seekers contain a wavelength-selective reflector and a defocusing reflector. However, the wavelength selective reflector allows only the SAL wavelength light to reach the defocusing reflector. Thus, the light to be imaged can be sharply focused onto the detector array and the SAL light can be de-focused to a larger spot. With this seeker, the light-absorbing regions of the detector array must absorb both the SAL wavelengths and the imager wavelengths. However, in these seekers approximately only half of the collected optical energy is detected by the SAL detector elements because the elements occupy only half of the overall area of the detector array.
Thus, known seekers have the SAL detector and/or the specific optical elements associated only with the SAL detector that obstruct the light path from, for example, the entrance dome (of the projectile) to the IIR detector. Accordingly, the overall optical system must have a larger diameter, otherwise the seeker would have lower imager sensitivity, as not all of the effective cross-sectional area at the dome can be used to collect light for the imager or for the SAL detector.